The present invention is related to control of a voltage-controlled oscillator, preferably in a phase locked loop.
Phase Locked Loops (PLL) are used, e.g. for obtaining an output clock frequency which follows an input clock frequency.
A phase locked loop can for example comprise a voltage-controlled oscillator (VCO), which generates a frequency depending on an input voltage. However, such an oscillator is not in practice ideal since the output frequency can be affected by, for example, the supply voltage, temperature, aging of components, humidity and atmospheric pressure. Therefore, the oscillator should be comprised in some type of control circuit. A number of different control circuits are possible.
In the U.S. patent U.S. Pat. No. 4,787,097, a phase locked loop having a feed back control circuit is shown.
In the U.S. patent U.S. Pat. No. 5,463,351, a phase locked loop having two parallel feed back control circuits, where either the first or the second of the control circuits is selected is shown.
In the U.S. patent U.S. Pat. No. 4,829,258, a phase locked loop having two feed back control circuits connected in series is shown.
The present invention is based on the understanding that there are two contradictory problems to solve in the design of a control circuit for a voltage controlled oscillator, which is to generate an output frequency which is to follow an input frequency.
The first problem is that normally it is not desired that the output frequency follows the input frequency exactly, jitter in the input frequency signal should be attenuated. This requires a relatively slow control circuit having a relatively low limit frequency.
The other problem is that a voltage controlled oscillator is affected by for example supply voltage, temperature, aging of components, humidity and atmospheric pressure. This requires a relatively fast control circuit having a relatively high limit frequency.
None of the documents described in the state of the art has been able to identify these two contradictory problems and has not been able to solve them.
The object of the present invention is to solve these two problems using a circuit a bit like a cascade circuit. A fast inner control circuit, having an inner controller and a stable oscillator is used to remove the noise in the voltage-controlled oscillator.
An outer slow control circuit having an outer controller is used in order for the output frequency to follow the input frequency, without following possible jitter in the input frequency.
Simplified, one could say that the inner control circuit is used to provide a stable xe2x80x9cbasic frequencyxe2x80x9d in the output frequency. The deviations in the input frequency from the basic frequency is then added to this basic frequency, resulting in that the output frequency follows the input frequency in a xe2x80x9csoftxe2x80x9d way.
The advantage of the invention is that noise in the voltage-controlled oscillator is removed at the same time as the input frequency is followed and that jitter in the input frequency is attenuated. Other advantages are good frequency and phase stability and that a large working range is obtained and that a low limit frequency (cut off frequency, modulation band width) can be used for attenuating the jitter in the input frequency.
In one embodiment phase meters are used for calculating phase differences, despite that the voltage controlled oscillator primarily controls the frequency. The problem with this is that it becomes necessary to consider both phase and frequency simultaneously. The embodiment relies on the understanding that a phase difference can be seen as an integration of frequency difference and thus either a differentiation or an integration must be provided at suitable locations. According to the embodiment an additional integration is provided in the outer control circuit making the outer control circuit generate an outer output signal consisting of a phase ramp. The advantage of this embodiment is that a simple useful circuit without unnecessary elements is obtained.
An alternative to this embodiment relies on the understanding that the frequency integrations in the inner control circuit will provide phase ramps, but that a phase meter only has a limited range, such as xe2x88x92180xc2x0 to 180xc2x0, which results in a phase jump of +360xc2x0 and xe2x88x92360xc2x0, respectively, at the interval boundaries. This problem is solved by making a corresponding compensation of xe2x88x92360xc2x0 and +360xc2x0, respectively, on the output signal from the outer controller.
In one embodiment the location of an addition and a subtraction according to A+(Bxe2x88x92C)=(A+B)xe2x88x92C is interchanged in the implementation. The advantage of this embodiment is that a simple circuit without any unnecessary elements is obtained.
One embodiment relies on the understanding that there may be problems if the input frequency signal disappears. This is solved according to the embodiments by that, simply put, the outer controller is xe2x80x9cfrozenxe2x80x9d so that the outer output signal provides the same phase ramp as before the signal disappeared. The advantage of this is that a relatively good output frequency is obtained even in the case of an error in the input frequency signal.
The invention will now be described in more detail by means of preferred embodiments and with reference to the accompanying drawings.